Light waves near infrared range (1,300 nm wavelength) are projected around the imaged structure and the reflected backscattered light signals form the images for analysis.

Optical coherence tomography has become established as an imaging modality in clinical ophthalmology and is finding applications in even gastrointestinal and dermatological arenas.

Its biggest application in cardiovascular medicine currently is in the coronary vasculature and to a lesser extent coronary graft assessment. Other areas of research include transplant vasculopathy, pulmonary hypertension and assessment of radial arteries as grafts.

Several commercially available systems are approved for clinical use. Due to the unrivalled resolution there has been fervent adoption of the technology in many catheterization laboratories around the world, though it must be stressed outcome data are still lacking.

Two types of OCT are available: Time Domain OCT (TD-OCT) and Fourier Domain (FD-OCT). Early systems employed TD-OCT; FD-OCT represented a significant improvement to allow for 10X faster pullback speed (20mm/sec) and obviated the need for proximal vessel occlusion. This played a crucial role in improving the ease of use of OCT.

Intracoronary FD-OCT was first demonstrated in swine studies in 2006, and then in 2008 in human in vivo studies.

In a small study FD-OCT and TD-OCT were performed consecutively and an FD-OCT study can be performed on average 7 minutes quicker and also yielded better images for analysis.

How is optical coherence tomography performed?

A prerequisite for image acquisition in OCT is a bloodless field. Blood, particularly the cellular components, causes scattering of light waves which precludes image acquisition. With TD-OCT, the bloodless field was achieved with proximal vessel occlusion and then injection with contrast or Ringers lactate. With FD-OCT, this can be achieved with non-occlusive contrast injection.

To perform OCT, conventional guiding catheters and coronary wires can be used. The monorail rapid exchange OCT catheter is compatible with a 6Fr guiding catheter, similar to IVUS.

As discussed, to ensure optimal image acquisition, sufficient flushing of blood is required, predicated on optimal guiding catheter engagement, particularly in large caliber vessels. Attention must be paid to guiding tip hemodynamics as forceful injections against plaque or during a ventricularized trace have been reported to cause dissection and adverse outcomes.

To prepare the OCT catheter, it needs to be flushed with 100% contrast. It is recommended that the system be calibrated (“offset”) with the imaging sensor between two fingers prior to being introduced into the guiding catheter.

The OCT catheter is passed distal to the point of interest. It is important to be aware the optical lens is situated more than 20mm from the distal radiopaque marker tip.

After the imaging lens is positioned distal to the point of interest, we routinely inject contrast at this point to ensure the correct position and adequate contrast flushing.

Both manual and automated contrast injection are conducive to adequate image acquisition. Often enough the operator will need to inject with much more force than conventionally, hence the importance to check guiding catheter tip hemodynamic trace prior to doing so.

For an automated system, we recommend 4mL/sec and 14mL for the left coronary system, 3mL/sec and 12mL for the right. With experience this can be tailored to the caliber and distribution of the arteries. It is not uncommon for a left dominant system with greater than 5mm diameter vessels to require 5-6 mL/sec of contrast.

Once contrast begins to opacify the vessel, the OCT machine operator can initiate pullback. This automated pullback is completed within 3 seconds.

It is paramount at this point to monitor for adverse effects such as arrhythmia and ischemia induced by aggressive contrast injection.

The acquired images can then be assessed for adequacy and clarity and if satisfactory the OCT catheter should be removed.

OCT data acquired can be analyzed immediately to guide decision making or be done so offline.

Indications and patient selection

The role for OCT in clinical applications has yet to be fully established.

The theoretical indication for OCT would be analogous to those for IVUS, in which additional intravascular information obtained could assist in clinical decision making – i.e. better delineation of angiographic uncertainty, dimension assessment, etc. Unlike IVUS, however, no outcome data are available on OCT.

It cannot be overemphasized however that the development of OCT is at a relatively nascent stage compared to IVUS and in the absence of definitive evidence of its application, and more importantly, outcome data, OCT should be viewed as a research tool.

It is therefore strongly recommended that OCT studies be enrolled on registries such as the Massachusetts General Hospital OCT registry to empower clinicians with prospective data that will establish the role of OCT in the clinical arena through rigorous follow up and data analysis.

In the absence of definitive guidelines, some general recommendations are possible as follows:

OCT recommended:

Research – given its unsurpassed resolution, with information such as stent coverage, neointima formation, OCT is highly suited for this purpose and arguably the gold standard for stent assessment nowadays.

Trial stent evaluation – e.g. ABSORB, LEADERS trials.

Plaque modification agents – response to treatment on OCT

OCT may be considered:

De novo lesion assessment – research is ongoing on such applications. It remains to be defined how such OCT can aid clinical decision making and how outcome can be improved or impacted upon – i.e. MLA guided decision-making analogous to IVUS derived parameters

PCI guidance – OCT can provide unprecedented information regarding the stent strut apposition, expansion and complications such as dissections post stent. Ideas such as OCT guided PCI remain to be investigated and proven of clinical benefit.

Delayed stent complications – in-stent restenosis or stent thrombosis. OCT has provided unprecedented information such as neoatherosclerosis and heterogeneity of in-stent restenotic tissue. This allows for important in vivo assessment for stent pathology such as in-stent restenosis or stent thrombosis. Whether this translates to better outcomes remains to be seen.

Novel applications – there have been publications of the use of OCT in spontaneous coronary artery dissection, chronic thromboembolic pulmonary hypertension and cardiac allograft vasculopathy. OCT appears useful in these situations.

For more information, the reader is encouraged to refer to an expert review paper by Lowe et al outlining the potential clinical applications of OCT.

Contraindications

OCT is contraindicated for any patient with a contraindication for coronary catheterization in general.

Caution should be used with any severe stenosis.

Caution should also be exercised in very tight lesions as the OCT catheter may itself occlude flow and contrast injection can lead to unacceptable trauma such as dissection.

OCT is unlikely useful in certain technical situations. These include: large vessels (particularly greater than 4mm), ostial lesions precluding satisfactory contrast clearance and very distal small vessels. Saphenous vein grafts are generally of large caliber and present a relative contraindication. However, two studies have demonstrated the feasibility of using OCT in assessment SVG culprit lesions in acute coronary syndromes and stable asymptomatic patients. As long as the vein graft can be sufficiently opacified with contrast injection, OCT is likely successful.

Some operators have been innovative in using wire biasing to image segments of large arteries (e.g. coronary wire into a large septal artery, to interrogate the septal side of a large proximal LAD).

Interpretation of results

OCT provides a wealth of information. The detail may appear overwhelming at first but interpretation of basic structures is relatively straightforward. Even interpretation of various de novo pathology or stent assessment requires a relatively quick learning curve.

A Normal coronary artery

The trilaminar structure (intima-media-adventitia) of the coronary artery is well defined as shown on Figure 1. The intima is defined as a bright line from the internal elastic lamina bordering the media internally. The media is a signal poor (dark) band. The adventitia is signal rich caused by the external elastic lamina. Intimal thickness increases with advancing age.

The intimal thickness is well assessed by OCT, whereas IVUS resolution is limited to intimal-media thickness (IMT). This spurned interests in OCT in following up intimal thickness in response to agents such as statin. Intimal hyperplasia appears as a homogenous signal rich thickening with no signal attenuation.

Atherosclerosis in vivo – atherosclerosis has been extensively examined by OCT studies for well over a decade.

Figure 1.

Figure 2.

Various coronary plaque types. Note the diffuse irregular edge of lipid plaque and contrast with the defined edge of calcium.

Figure 3.

Acute coronary syndrome plaque rupture with thrombus.

Stent assessment

The OCT resolution enables its utility in assessment stent apposition as well as stent strut coverage. Using manufacturer’s information of strut thickness, neointimal hyperplasia can be measured.

Apposition can be defined as embedded struts, protruded struts (intimal contact but not covered with intima) to malapposition (no intimal contact). (Figure 4).

Struts can also be deemed covered or uncovered. Issues such as late lumen loss are well elucidated by OCT for stent follow up.

Stent malapposition with thrombus (Figure 5) – stent pathology shown in acute coronary syndromes. Gross stent malapposition is well known as a cause of stent thrombosis. In-stent restenotic tissue has been seen to have neovascularization and “neoatherosclerosis” plaque rupture (Figure 6) with overlying thrombus.

Additionally, OCT is easier to use and the findings easier to interpret. Studies have shown less inter-observer variability in OCT interpretations than IVUS interpretations.

Despite its obvious advantages, IVUS has been in use for almost 20 years and OCT is in its relative infancy and the available data of OCT clinical applications remain relatively limited.

Nonetheless it is foreseeable that for the majority of PCI cases, FD-OCT can provide useful information. If only one intravascular imaging modality is to be chosen in a newly built facility, OCT is probably the modality of choice.

There have been several in vivo studies comparing IVUS and OCT in various clinical scenarios:

One study of 19 lesions in 15 patients found generally acceptable correlations in measurements between IVUS and OCT in minimal luminal area but slightly weaker correlations in the proximal and distal reference diameters. This may have implications in using IVUS derived parameters in decision making for revascularization.

In a study of 30 patients with AMI, OCT was able to detect plaque rupture, fibrous cap erosion and thrombus more accurately than IVUS.

In a seminal paper, OCT was able to identify intimal hyperplasia and echolucent regions, which may correspond to lipid pool, more frequently than IVUS.

Complications and their management

The feasibility and safety of OCT have been well established in various clinical scenarios in registries and trials. Despite the need for high-flow-rate contrast injection during image acquisition, complications are rare. The development of a non-occlusive FD-OCT system has further minimized the risks related to contrast injection.

There have been no large FD-OCT registries but from smaller studies the overall risk of complication is much less than 1%. At one center where OCT has been utilized for more than 10 years, there have been no complications at all.

OCT has been assessed in patients with stable and unstable coronary artery disease and proven safe.

With experience, most operators will be able to achieve a “one-run” routine to avoid unnecessary non-contributory runs.

What have we learnt from OCT research so far?

The biggest advantage of OCT is the possibility of histological equivalent information gained in vivo rather than post mortem intracoronary pathological assessment.

In vivo plaque characterization

The normal vessel wall is seen as a trilaminar structure of intima, media and adventitia. A variety of pathologies have been reported on OCT, including neointimal thickening, calcification, thrombus (red vs. white thrombus) and plaque ruptures.

Studies have shown the use of OCT in differentiating fibrous, fibrocalcific and lipid-rich plaques.

Pathological studies have previously documented the importance of thin capped fibroatheroma (TCFA) in coronary artery disease. TCFA, defined as the triad of a lipid core, a fibrous cap with a thickness of less than 65 microns and inflammatory cell infiltration in the fibrous cap, is beyond the resolution of IVUS and OCT has been instrumental in evaluating the role of TCFA in acute coronary syndromes and in stable angina.

OCT plaque characterization in acute coronary syndromes has been examined in several studies focusing on the plaque differences between various modes of presentation, culprit and nonculprit lesions, NSTEMIs and STEMIs.

The idea of vulnerable plaques, the crux to the holy grail of infarct prevention, has also been studied with OCT. One such marker, neovascularization, is seen as microchannels within plaques. OCT seems most suited of all intravascular imaging modalities to identify most of the features in vulnerable plaques such as plaque fissures, superficial platelet clumps on plaques, TCFA and inflammatory cell infiltration.

One limitation of using OCT in plaque characterization is differentiating calcium from lipid, which has been identified in studies as most likely leading to inaccurate OCT interpretation.

Multiple case reports and case series have been focused on its role in spontaneous coronary artery dissection. OCT demonstrates well the pathology of dissection and seems useful in PCI guidance.

PCI guidance and immediate assessment

One group used OCT to guide PCI. It seems feasible to assess the results of lesion coverage with stents longitudinally, and stent strut apposition and complications such as stent edge dissection.

The use of OCT to determine necessity of revascularization, such as that based on minimal luminal area, needs further evaluation and at this stage lacks data. Transferring data from IVUS does not seem appropriate.

Better understanding of plaque composition may allow more preemptive treatment. For instance, a lesion with a larger lipid core has been shown to be associated with a higher periprocedural cardiac biomarker release. Whether this preemptive knowledge translates into better care remains to be seen.

To temper the enthusiasm, it is sobering that IVUS guided PCI was not conclusively associated with better outcomes in randomized controlled trials.

There are some data on the use of OCT in bifurcation lesions in the assessment of the neocarina, lesion coverage and using 3D to guide side branch wiring.

Therapeutic (pharmacological/PCI) follow up and research

Given the resolution, OCT is well established to be the gold standard in coronary stent research. Areas of interest include stent strut coverage, neointima formation, malapposition and in-stent restenotic tissue.

In-stent neoatherosclerosis and neovascularization has been demonstrated in both bare metal and drug-eluting stents and they seem to differ in composition and time points.

Various trials assessing new stents are using OCT endpoints for follow up.

OCT was used in one study to assess the thickness of neointima in response to statin therapy. This may provide a new means to assess other plaque modifying agents.

Where do you see the future of OCT?

More data is needed to cement the role of OCT in routine interventional practice and recently commenced multicenter registries will help define this.

There are further technical advancements in the future including histological equivalent (1 micron) OCT and incorporated 3D assessment.

Outcome data is imperative to justify its use and it is likely cost effectiveness data will be useful for it to be incorporated in various guidelines.